The purpose of the study was to explore the reflective process of pre-service English teachers by analyzing critical incidents entered in their teaching journals during a fourweek student teaching practicum. Thirty-three student teachers were asked to report critical incidents in their journals once a week, especially focusing on „lesson breakdowns‟ or „events that deserve critical significance‟ during a lesson. 133 journal entries were collected and a total of 104 critical incidents were analyzed to see some aspect of secondary classroom teaching and the reflective processes that student teachers went through right after the incidents. From the content analysis, it was found that many critical incidents happened during the initial stage of lesson and were mostly caused by student teachers‟ lack of experiences in either time management or material preparation tailored to students‟ level. By describing and analyzing critical incidents, student teachers became more aware of their assumptions about language teaching and learning and tried to transfer knowledge into real practices. A survey administered after the practicum also revealed that self-observation of critical incidents in their own classroom helped them to generate powerful insights about teaching and to develop reflective teachers.